1. Field of the Invention
The present invention relates generally to electron source modules adaptable for use in electronic equipment, and more particularly to multilayer thin-film electron emitter devices as well as application equipment employing the same. The invention also relates to metal-insulator-metal (MIM) or metal-insulator-semiconductor (MIS) three-layer thin-film electron emitters and their application equipment including, but not limited to, display apparatus and electron beam (EB) lithography apparatus.
2. Description of the Related Art
Conventionally, thin-film electron emitter modules come with three-layered structures such as the MIM structure or MIS structure consisting of the lamination of three thin films: an upper or "top" electrode, an intermediate or "middle" insulative layer, and a lower or "base" electrode. Upon application of an external potential between the top and base electrodes with the top electrode being positive in polarity, these MIM and MIS electron emitters operate to emit or liberate electrons from the surface of the top electrode into a vacuum. In the past, various types of electron emitters have been proposed, including those of the MIM type which employ metal for its top and base electrodes, and those of the MIS type making use of a semiconductor for at least one of these electrodes. One typical MIM electron emitter has been disclosed in, for example, Published Unexamined Japanese Patent Application (PUJPA) No. 7-65710.
Principally, the operation of thin-film electron emitters is as follows. Upon application of a drive voltage between the top and base electrodes while potentially setting an electrical field within an insulative or dielectric layer sandwiched therebetween at 1 to 10 megavolts per centimeter (MV/cm) or greater, electrons which reside near or around the Fermi level in the base electrode are potentially activated to penetrate by tunnel effect through a barrier entering by injection into the conduction band of the dielectric layer; thereafter, these electrons are accelerated to be further injected into the conduction band of the top electrode thereby behaving as so-called "hot" electrons. Of those hot electrons, certain ones which have energy greater than the work function .phi. of the top electrode are then released and emitted into the vacuum. By way of example, electron emission based on the above principle has been observed in a three-layer lamination electrode structure of Au-Al.sub.2 O.sub.3 -Al. This type of electron emitter offers in nature advantages as to performance and reliability. For example one advantage is that the electron emission characteristics will be substantially kept constant even when the work function .phi. varies due to occurrence of contamination on the surface of the top electrode caused by absorption of ambient gas therein. This leads to the exception that such devices are highly advanced electron emitters which will become more and more important in several applications.
Unfortunately, the prior art electron emitters are faced with a serious problem in that, due to the necessity of applying to the dielectric layer a relatively strong electric field the intensity of which is as high as 1 to 10 MV/cm, degradation will possibly occur in the dielectric layer resulting in the so-called "forming" phenomenon as taught by PUJPA No. 7-226146, for example. This, in turn, disadvantageously leads to occurrence and mixture of noises in the resultant flow of emitted electrons or "current" while simultaneously resulting in electrical failure or breakdown which can lead to destruction of thin-film electron emitters in the worst case.
The language "insulative of dielectric layer" used herein includes semiconductors if their resistivity is high enough to tolerate the high electric field. An example of such thin-film electron emitters consists of silicon for the base electrode, porous silicon for insulator, and metal for the top electrode, as is described in the Japanese Journal of Applied Physics, Vol. 34, Part 2, No. 6A, pp. L705-L707 (1995).